Leaning vehicles having more than one front or rear wheels require a frame that is pivotally connected to the two-wheel suspension assemblies to permit the vehicle to lean. One such vehicle is disclosed in PCT Publication No. WO 2011/059456 A1 (the '456 publication), published on May 19, 2011. The vehicle in the '456 publication has a two front wheels and a single rear wheel.
FIGS. 1 and 2 illustrate a frame 200, shock tower 202 and a front right suspension assembly 204 of a vehicle of the type described in the '456 publication. The front left suspension assembly of this vehicle is a mirror image of the front right suspension assembly 204 and as such will not be described in detail herein.
As can be seen in FIG. 1, the frame 200 is pivotally connected to the shock tower 202 about a frame leaning axis 206. The front right suspension assembly 204 has a lower suspensions arm 208, a leaning rod 210 and a shock absorber 212. The lower suspension arm 208 is pivotally connected at one end to the frame 200 about a pivoting axis 214 and is pivotally connected at the other end to a kingpin 216 about a tilting axis 217. The kingpin 216 has a knuckle 219 pivotally disposed thereon. The knuckle 219 supports the front right wheel thereon. The inner end of the lower suspension arm of the front left suspension assembly is similarly connected to the frame 200 about a pivoting axis 218. As can be seen in FIG. 1, when the frame 200 is in the upright position, the pivoting axes 214, 218 are disposed to the right and left of the frame leaning axis 206 respectively and are located vertically higher than the frame leaning axis 206. The leaning rod 210 is pivotally connected to the frame 200 at one end and to the kingpin 216 at the other end. The shock absorber 212 is connected to the lower suspension arm 208 at its lower end and to the shock tower 202 at its upper end.
When the vehicle turns, the frame 200 and, as a result, the wheels lean toward the inside of the turn. When making a right turn as shown in FIG. 2, the frame 200 pivots toward the right about the frame leaning axis 206 and the wheels also pivot toward the right as would be understood from the pivoting of the kingpin 216 about the tilting axis 217.
Additional details regarding a vehicle of this type and the manner in which it leans can be found in the '456 publication.
When turning, the frame 200 and other components of the vehicle need to be sufficiently leaning such that the lateral forces between the tires and the ground are sufficient to prevent the vehicle from falling over.
As can be seen by comparing FIG. 1 to FIG. 2A, due to the manner in which the lower suspension arms 208 are connected to the frame 200, when the frame 200 leans, the pivoting axes 214, 218 are displaced from the positions they occupy when in the upright position of the frame 200. This displacement of the pivoting axes 214, 218 also causes a displacement of the frame leaning axis 206 and thus causes the center of gravity (CG) to travel along trajectory 1 (FIG. 2A) which resembles an arc rather than a constant radius as in trajectory 2 (FIG. 2A).
FIG. 2A shows two different trajectories of a CG during a turn and the displacement of the center of pressure (CP) of the tires, or its equivalent in the case of a three-wheel vehicle, on the ground. Trajectory 1 represents the movement of a CG of a vehicle and of a corresponding center of pressure CP 1 with the suspension of FIGS. 1 and 2 and trajectory 2 represents the movement of a CG and a corresponding center of pressure CP2 of a motorcycle having two in-line wheels.
As can be seen in FIG. 2, when the frame 200 leans to the right, the frame leaning axis 206, and the rest of the leaning components, are displaced upward and to the right from the positions they occupy when the frame 200 is upright (illustrated by axis 206′ in FIG. 2 for the leaning axis 206), similar to that of trajectory 1 of FIG. 2a. As can also be seen in FIG. 2A, the CP of a vehicle with the suspension geometry of FIG. 1 moves laterally and thus the effective lean angle (the angle between a line passing through the CG and the CP with respect to vertical) has been reduced. As can be seen in FIG. 2A, the effective lean angle A is less than the effective lean angle B. The lateral g-forces generated is a ratio of the horizontal and vertical distances between the CP and the vehicle's CG. Therefore, trajectory 1 generates less lateral g-forces than trajectory 2 even though both vehicle frames are equally leaned with respect to vertical. As such, for two vehicles, one with the geometry of FIG. 1 and the other with inline wheels, to travel through the same trajectory with the same travel characteristics, the vehicle with the geometry of FIG. 1 (trajectory 1) would have to lean further with respect to vertical.
Thus, there is a need for a leaning vehicle having at least three wheels that requires less leaning to produce a desired lateral g-force.
Furthermore, it may be desirable under certain conditions to prevent the frame of a leaning vehicle from leaning. Examples of such conditions include when the vehicle is parked or operating at low speeds.
Thus, there is a need for a leaning vehicle in which the frame can be prevented from leaning.
In a vehicle that does not lean, the shock absorbers are connected to the frame. As such, the unsprung mass (i.e. the wheels, brakes and other elements connected to the frame via the suspension system) is much smaller than the sprung mass (i.e. the frame and other elements supported by the suspension system). As such, when a wheel encounters a bump for example, the sprung mass is sufficiently large to cause the shock absorber to compress.
However, for a leaning vehicle of the type disclosed in the '456 publication, the shock absorbers are connected to the shock tower as described above. As would be understood, the mass of the shock tower is much smaller than the sprung mass of the non-leaning vehicle described above (all things being equal except for the leaning aspect of the vehicle) thus the system has very low damping characteristics again the rotation of the shock tower. When the wheels go over large bumps or depressions (i.e. high amplitude movement), the shock absorbers react essentially as they would in a non-leaning vehicle where the shock absorbers are connected to the frame. When the wheels go over small bumps or depressions (i.e. low amplitude movement/higher frequency) some issues can arise however. The forces generated by such low amplitude movement of the wheels can be too small for the tires to absorb and for the stiffness of the shock absorber to be overcome (i.e. the shock absorber does not compress). The mass of the shock tower can also be too small to resist the force being transferred through the shock absorber. As a result, the shock tower, the two shock absorbers, the suspension arms and the wheels essentially react as if they were a single rigid element. When the left wheel goes over a small bump for example, the force is transferred from the left wheel to the left shock absorber, then to the shock tower, and since the left shock absorber does not compress, the force is then transferred from the shock tower to the right shock absorber and to the right wheel. Since the right shock absorber also does not compress, the tire of the right wheel compresses, then springs back thereby transferring the force back through the shock tower to the left shock absorber and the left wheel, and the process is repeated. This oscillation of the force can eventually lead to the wheels oscillating up and down, which, when the vehicle is manoeuvring through a turn, is sometimes referred to as “wheel hop” since this oscillation can cause the tires to momentarily loose contact with the ground and be pushed toward the outside of the turn. As would be understood, “wheel hop” is not desirable.
Thus, there is a need for a leaning vehicle in which the shock absorbers are attached to the shock tower that addresses the “wheel hop” phenomenon.